Page 166 - Programmable Logic Controllers, Fifth Edition

7.6 Cascading Timers
each using two hardwired on-delay timers. The operation
of the circuit can be summarized as follows:
When one timer’s output triggers another timer’s input,
those timers are referred to as cascaded. Timers can be • Motor starter coil M1 is energized when the
interconnected, or cascaded, to satisfy a number of logic momentary start pushbutton PB2 is actuated.
control functions. • As a result, motor 1 starts, contact M1-1 closes to
Figure 7-31 shows how three motors can be started seal in M1, and timer coil TD1 is energized to begin
automatically in sequence with a 20 s time delay between the first time-delay period.
• After the preset time period of 20 s, TD1-1 contact
closes to energize motor starter coil M2.
Start
L1 Stop PB2 L2 • As a result, motor 2 starts and timer coil TD2
PB1 OL is energized to begin the second time-delay
M1 period.

M1-1 • After the preset time period of 20 s, TD2-1 contact
TD1 closes to energize motor starter coil M3, and so
motor 3 starts.
TD1-1 OL
M2 Figure 7-32 shows an equivalent PLC program of the
hardwired sequential time-delayed motor-starting circuit.
(20 s) Two programmed on-delay timers are cascaded together
TD2
to obtain the same logic as the original hardwired timer
TD2-1 OL relay circuit. Note that the output of timer T4:1 is used to
M3 control the input logic to timer T4:2.
Reciprocating timers are defined as timing functions
(20 s) where the output of one timer is used to reset the input
Figure 7-31 Hardwired sequential time-delayed motor- of a second timer, each resetting the other. These types
starting circuit. of timers are used in situations where a constant cycling

Inputs Ladder logic program Outputs
PB1 PB2 M1
L1 Stop L2
OL
PB1
M1 M1
Start
OL
PB2 M2
TON
M1 TIMER ON DELAY OL
Timer T4:1 EN M3
Preset 20
Accumulated 0 DN

T4:1/DN M2

TON
T4:1/DN
TIMER ON DELAY
Timer T4:2 EN
Preset 20
Accumulated 0 DN
T4:2/DN M3

Figure 7-32 Equivalent PLC program of the sequential time-delayed motor-starting circuit.

Programming Timers Chapter 7 147

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